The plant surface serves as an interface to the environment. Many substances from the outside are taken up by the plant and many internal products are released. In addition, some of these substances produced or taken up by the plant are also stored in surface structures. The interactions between the plant and its environment take place on the aerial surface such as the shoot, leaves, flowers, etc. as well as on the surface of the root. The surface of plants is built by the epidermis, a tissue in which highly specialized cells are formed to accomplish the various above sketched functions. Examples of these specialized cell types are e.g. plant hairs (trichomes), which are involved in protection against insects or other pathogens by either secretion of substances or steric hindrance, root hairs which mediate nutrient and water uptake, or stomata which regulate gas exchange. Trichomes are single-celled hairs that during wild-type maturation undergo an average of four rounds of endoreduplication, leading to a characteristic 3-4-branched cell with a DNA content of approximately 32C. See Hulskamp et al., Int. Rev. Cytol., 186, 147-178 (1999) and Traas et al., Curr. Opin. Plant Biol., 1, 498-503 (1998). Mutant trichomes with a lesser DNA content (e.g., 16C) are smaller and have fewer branches, whereas trichomes with a higher DNA content (e.g., 64C) are larger and develop more branches.
Due to their metabolic activities, many of the epidermal cells of various plants are of economical interest. An alteration of the cell number, size, or other cellular parameters can influence the metabolic power and thus their economical use.